How did Shiva become god?

By concept, TriDevas( Brahma, Vishnu & Shiva) are fundamental and no one created them. However, Padmapurana speaks of Vishnu creating Shiva. However Padmapurana may not be a real purana. It was created by some cultists hoping to spread their influence.The Puranas are basically stories created to make people understand the truth of Vedas. These stories differ from Puranas to Puranas as they are not the absolute truth.Any person hoping to worship his chosen form-Brahma,Vishnu or Mahesh(Shiva ) reads the particular Purana to increase his or her devotion. As such there is no need to compare and classify the Puranas as done by Padmapurana.